System and apparatus for simultaneous control of the levels of signals being fed along separate paths

ABSTRACT

A plurality of light-sensitive variable resistors are each connected in a different path along which is fed an electrical signal whose level is to be controlled. These light-sensitive variable resistors are supplied with the required light by their respective optical fibers extending from a common light source. The intensity of the light thus delivered from the light source to the light-sensitive variable resistors is regulated, as by an apertured shutter, to cause the variable resistors to offer correspondingly regulated electrical resistance to the passage therethrough of the respective electrical signals. This system of signal level control is described more specifically as adapted for the expression control of an electronic keyboard musical instrument such as an electronic organ furnished with a plurality of keyboards. There is further disclosed herein the actual construction of an expression control apparatus based upon the signal level control system.

United States Patent 1191 Nakata et a].

1451 Nov, 18, 1975 SYSTEM AND APPARATUS FOR 3,557,295 1/1971 Adachi 84/118 SI LT EO CONTROL OF THE 3,612,469 11/1971 Ada chi. 1 v 7. 84/109 X SEPARATE PATHS 3,800,058 3/1974 Bartok et al. 84/1.18 Inventors: Akira Nakata Ts boi ch Okamoto et a]. v .1 Hiroshi Amano, 1923-60 Nishijama-cho, both of Hamamatsu, Pr'mary Emmmer stephen Tomsky Japan Assistant ExaminerStanley J. Witkowski Filed: Nov- 1974 Attorney, Agent, or Firm-Kenneth S. Goldfarb [21] App]? No: 528,468 [57] ABSTRACT Related US Application Data A plurality of light-sensitive variable resistohrs are each [62] Division of Sen No 351062 May 3, 1973, connected m a different path along who is fed an abandoned electrical signal whose level is to be controlled. These light-sensitive variable resistors are supplied with the [30] Foreign Application Priority Data required hght by their respective optical fibers extend- J I I 6 977 M I n 47 80485 mg from a common light source. The 1ntens1ty of the u y 7 light thus delivered from the light source to the lightsensitive variable resistors is regulated, as by an aperg 3 tured shutter, to cause the variable resistors to offer [58] d 84/1 09 l I 17 1 l8 correspondingly regulated electrical resistance to the e 0 are id passage therethrough of the respective electrical signals. This system of signal level control is described more specifically as adapted for the expression control [56] References Cned of an electronic keyboard musical instrument such as UNITED STATES PATENTS an electronic organ furnished with a plurality of key- 3,045,522 7/1962 Markowitz et a]. 84/ 1127 boards. There is further disclosed herein the actual 3.049.957 8/1962 Krug 84/109 construction of an expression control apparatus based Meyer 1 t. upon the ignal level control ystenm 3,461,217 8/1969 Omura et al... 84/DIG. l9 3,483,304 12/1969 Jenny 84/1.18 7 Claims, 7 Drawlng Figures TONE E 1 SW6 COLORING AMP CKT CKT 13a 11a 1 2a 12a TONE o??? SW6 COLORING CKT CKT CK T 1b iib 12b SW6 TONE CKT C(ELKOTRING 1 1c lc U.S. Patent Nov. 18, 1975 Sheet 2 of 4 TONE GEN CKT FIG. 2

II III 16a 5W6 C TONE OLORING O T 11a 12a H m 16b 5W6 TONE j LORING CKT 1 5W6 TONE CKT COLORING AMP elm AMP aw FIG. 3

u m qea SW6 CJL O IiEN AMP G CKST CKKT 13a 8 g 11a 12a 14a 15 H In 1% 1 TONE TONE GEN COLORING AMP CKST S CKT r 13b g S 10 11b 12b 12b 15b II III 1 5W6 TONE CKT co zme S 11c L' c SYSTEM AND APPARATUS FOR SIMULTANEOUS CONTROL OF THE LEVELS OF SIGNALS BEING FED ALONG SEPARATE PATHS This is a division, of application Ser. No. 357,062, filed May 3, 1973, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to an improved system for simultaneous control of the levels of electrical signals being fed along separate paths in parallel relationship, and in particular to such a signal level control system as adapted specifically for the expression control of an electronic musical instrument such as an electronic organ. The invention is also specifically directed to the mechanical configuration of the expression control apparatus in strict accordance with the improved signal level control system.

The expression control has been widely used in electronic organs or like musical instruments to give greater expression to musical performance. In a typical example of such known electronic organs incorporating the expression control, any of the keys of one of the keyboards, for example the upper keyboard, may be depressed to cause conduction through a corresponding one of switching circuits, whereupon the desired output signal from a tone generator circuit is introduced into a tone coloring circuit corresponding to the upper keyboard via the conductive switching circuit. At the tone coloring circuit the output signal from the tone generator circuit is filtered so as to represent the tone color or timbre of any desired musical instrument such for example as a flute or violin. The thus-filtered output signal from the tone coloring circuit is further fed through an expression control circuit and amplifier circuit and is translated into audible sound by a loudspeaker connected to the amplifier circuit.

A similar procedure of sound production also takes place when any key of the lower keyboard is depressed. However, the sound production systems associated with the respective keyboards of the electronic musical instrument may either share one and the same expression control circuit or may be equipped with their own exclusive expression control circuits.

In either case the configuration of the prior art expression control circuit has mostly been such that a light-sensitive variable resistor such as a cadmium-sulphide photo-resistor is positioned opposite to a light source, with an apertured shutter interposed therebetween and operatively coupled to an expression pedal for regulating the intensity of the light incident upon the light-sensitive variable resistor.

Thus, when the expression pedal is depressed to a desired degree, as to increase or decrease the intensity-of the light irradiating the light-sensitive variable resistor through the apertured shutter, the level of the signal traveling as aforesaid from the tone coloring circuit to the amplifier circuit is regulated to cause. the corresponding change in the volume of the sound emitted from the loudspeaker. More expressive musical performance can thus be obtained by proper actuation of the expression pedal.

As is well known, however, the more sophisticated electronic organs are usually equipped with a footoperated pedal keyboard in addition to the upper and lower digital keyboards. The pedal keyboard has its own sound production system just like those of the upper and lower keyboards described previously.

2 Highly complex arrangement has been required in case the expression control circuit incorporated in the sound production system of the pedal keyboard is made operable by the same expression pedal as for those of the upper and lower keyboards.

Although this problem may be circumvented by the provision of as many expression pedals as the total number of the sound production systems employed in the electronic musical instrument, the playing of the instrument will then become so difficult that the desired expressive performance will not easily be realized. It is believed that similar difficulties are encountered in many other fields of technology.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a novel and improved signal level control system particularly well adapted for the expression control of an electronic organ or like musical instrument, among other applications.

Another object of this invention is to provide a signal level control system utilizing optical fibers to lead the light from the light source to the light-sensitive variable resistor, thus making it possible to use the electrical wire of minimum length for feeding electrical signals to be controlled, for the benefit of reducing hum or other noises.

Still another object of the invention is to provide a signal level control system making use of optical fibers for simultaneous, centralized control of the levels of signals being fed along different paths in substantially parallel relationship, such that when adapted for the expression control of an electronic organ or like musical instrument comprising a plurality of keyboards or a plurality of highly involved sound production systems, it requires only one expression pedal and further highly simplifies the overall configuration of the expression control circuitry.

It is also an object of this invention to provide an expression control apparatus for use in an electronic organ or like musical instrument, the. apparatus being closely based upon the above described system of the invention for simultaneous control, by means of one expression pedal only, of the levels of the signals being delivered from a plurality of respective tone coloring circuits to amplifier circuits of the electronic musical instrument.

With these objects in view and the other objects hereinafter pointed out, this invention provides a signal level control system including a plurality of light-sensitive variable resistors each connected in a different path along which is fed an electrical signal whose level is to be controlled. These light-sensitive variable resistors are caused to offer variable electrical resistance to the passage therethrough of the respective electrical signals by being supplied with light of variable intensity through optical fibers extending from a common light source. The intensity of the light thus directed from the light source to the light-sensitive variable resistors by the respective optical fibers is regulated by means typically including an apertured shutter.

In case the above summarized system is adapted for the expression control of an electronic organ or like musical instrument, the apertured shutter can be operatively associated with an expression pedal so that the electrical resistance offered by the light-sensitive variable resistors, which in this case may be connected between the tone coloring circuits and amplifier circuits 3 respectively of the electronic musical instrument, is regulated in accordance with the degree to which the expression pedal is depressed. The volume of the sounds to be emitted from'loudspeakers connected to the respective amplifier circuits can thus be regulated by the single expression pedal.

The features which are believed to be novel and, characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and mode of operation, together with the further objects and advantages thereof, will be best understood from the following description taken in connection with the accompanying drawings which illustrate, by way of example only, some preferred embodiments of the invention and in which like reference characters designate like parts throughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a combined schematic and block diagram showing a signal level control system as adapted for the expression control of an electronic organ by way of a preferred embodiment of the invention; I

FIG. 2 is a similar diagram showing another preferred embodiment of the invention;

FIG. 3 also is a similar diagram showing still another preferred embodiment of the invention;

FIG. 4 is a back elevational view of an expression control apparatus constructed in accordance with the concepts of the invention;

FIG. 5 is a vertical sectional view taken substantially along the plane of line 55 in FIG. 4;

FIG. 6 is a side elevational view of an apertured shutter used in the expression control apparatus of FIGS. 4 and 5; and

FIG. 7 is a horizontal sectional view taken substantially along the plane of line 77 in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will now be described more specifically with reference to FIG. 1 as adapted for the expression control of an electronic organ or the like incorporating three separate systems of sound production by way of example. The reference numeral 10 in the drawing denotes a tone generator circuit composed of three groups of tone generators associated respectively with the upper and lower manual keyboards and the pedal keyboard of the electronic musical instrument. The three groups of tone generators in the circuit 10 are connected through their respective groups of'switching circuits 11a to 110, tone coloring circuits 12a to 120, light-sensitive variable resistors 13a to 13c such for example as cadmium-sulphide photoresistors, and amplifier circuits 14a to 140, to loudspeakers 15a to 150 respectively.

The groups of switching circuits 11a to 11c are electrically connected to the upper manual keyboard 160, the lower manual keyboard 16b, and the pedal keyboard l6c', respectively, in such a manner that the respective switching circuits in each group are rendered conductive upon depression of the corresponding keys of the manual keyboard 16a or 16b or the pedal keyboard l6c. The light-sensitive variable resistors 13a to 13c are each caused to offer variable electrical resistance in accordance with the intensity of light delivered through optical fibers 17a to 17c respectively. The

open ends of these optical fibers may be coupled directly to the respective light-sensitive variable resistors or may simply be positioned opposite thereto.

If desired,"the,.other open endsof the-optical fibers 17a to 170 may be united or combined together as indicated at 17d in the drawing, the combined ends 17d of the optical fibers being open to a light source chamber, not specifically shown in FIG. 1 but later indicated more clearly with reference toFIG. 5, of an expression control .apparatus generallydesignated by the reference numeral 18. As is well known to those in the art, the expression control apparatus 18 includes a light source 19 such as a lamp electrically connected to a DC power supply 20. A light filter 21 and a shutter 22 are positioned with prescribed spacings between the light source 19 and the combined open ends 17d of the optical fibers l7ato 17c. I

The light filter 21, made for example of frosted glass, is adapted to permit only the light of predetermined wavelength and of uniformized energy distribution to pass into the combined open ends of the optical fibers. The'provision of this light filter is not of absolute necessity. Although not seen in this drawing, it is assumed that the shutter 22 is apertured in accordance with the prior art to regulate the intensity of the light delivered therethrough into the combined open ends of the optical fibers. The apertured shutter 22 is operatively associated with the expression pedal 23 of the electronic musical instrument, in such a manner that the shutter is moved relative to the light source 19 and the combined open ends of the optical fibers 17a to 170 as the expression pedal is depressed, as hereinafter referred to in further detail. I

Proceeding to the description of a mode of operation of this firstpreferred embodiment of the invention, the light distributed from the light source 19 is to pass through the perture, not shown, of the shutter 22 and through the filter 21, if any, into the respective optical fibers 17a to 17c through their combined open ends 17d and is thus conveyed by internal total reflection to the respective light-sensitive variable resistors 13a to 13c.

More specifically, if the expression pedal 23 is depressed slightly, the apertured shutter 22 operatively associated therewith is caused to move relative to the light source 19 and the combined optical fiber ends 17d to permitthe light of correspondingly low intensity to be delivered to the light-sensitive variable resistors 13a to through their respective optical fibers 17a to 170. Consequently, these light-sensitive variable resistors offer relatively high electrical resistance to the passage therethrough of the output signals from the respective tone coloring circuits 12a to 12c, so that'the sounds emitted from the loudspeakers 15a to 15c are of extremely low volume, ,if audible at all.

If the expression pedal is depressed to an intermediate degree, then the apertured shutter permits the light of intermediate intensity to be delivered to the lightsensitive variable resistors through their respective optical fibers. Thereupon the resistance offered by the light-sensitive variable resistors drops to an intermediate degree thereby attenuating the output signals from the respective tone coloring circuits to the same degree. The loudspeakers are thus caused to produce the sounds with an intermediate degree of intensity.

Upon full depression of the expression pedal, the apertured shutter is caused tomove to such a position relative tothe light source and.the combined optical fiber ends that the light given off by the former is received with its substantially full intensity by the latter. The resistance offered by the light-sensitive variable resistors is now caused to drop to its predetermined lower limit, with result that the output signals from the respective tone coloring circuits 12a to 12c are permitted to pass substantially unattenuated through the respective lightscnsitive variable resistors to the corresponding amplifier circuits 14a to 14c. The volume of the sounds emitted from the loudspeakers a to 150 is thus maximized.

Hence, if the expression pedal 23 is depressed to a desired degree while at the same time any one or more of the keys of the manual keyboards 16a and 16b and the pedal keyboard [60 are depressed to cause conduction through the corresponding one or more of the switching circuits in their groups 11a to He, then the tone or tones corresponding to the depressed key or keys are produced from the corresponding one or more of the loudspeakers 15a to 15c with the desired degree of intensity. It is thus possible to make simultaneous control of the signal levels in the several separate systems of sound production by one and the same expression control apparatus 18. I

It will now be understood that the invention is applicable to any electronic organ or like musical instrument incorporating either one system of sound production or, as specifically set forth in the following table by way of example, a multiplicity of such systems:

Tone Coloring Right-hand Circuit A Tremolo Loudspeaker Under Tone Coloring Left-hand Keyboard Circuit B Tremolo Loudspeaker Tone Coloring Right-hand Circuit C Non-tremolo Loudspeaker Lower Tone Coloring Left-hand Keyboard Circuit D Tremolo Loudspeaker Pedal Tone Coloring Central Keyboard Circuit E Non-tremolo Loudspeaker By connecting the light-sensitive variable resistors 13 to the outputs of the respective tone coloring circuits A to E, and by regulating the electrical resistance offered by these variable resistors by irradiating the same with the light of controlled intensity delivered through the optical fibers 17, it is possible to make simultaneous control of the signal levels in all the complex sound production systems of the above table which permit highly expressive musical performance.

In another preferred embodiment of the invention illustrated in FIG. 2, the light-sensitive variable resistors 13a to 13c are incorporated within the expression control apparatus 18 and are placed opposite to one open end of each of the optical fibers 17a to 170 through the shutter 22 operatively associated with the expression pedal 23. The other ends of the optical fibers-are united or combined together as indicated at 17d and are placed opposite to the light source 19 through the light filter 21. The outputs of the tone coloring circuits 12a to 126 are connected to the respective light-sensitive variable resistors 13a to 130 and thence to the inputs of the amplifier circuits 14a to 14c as will be apparent from the drawing. 7

The second preferred embodiment of the invention being configured substantially as the foregoing, the expression pedal 23 may be depressed simultaneously .with the depression of any one the manual keyboards 16a and l6b and the pedal keyor more of the keys of board 166 of the electronic musical instrument. As previously set forth in connection with the embodiment of FIG. 1, the intensity of the light delivered from the light source 19 to the light-sensitive variable resistors 13a to 130 through the light filter 21, the optical fibers 17a to 17c, and the apertures 22a to 220 of the shutter 22, is regulated in accordance withthe degree to which the expression pedal 23 is depressed. Since the resistance of the light-sensitive variable resistors 13a to 130 is subject to change depending upon the intensity of the light delivered thereto as above stated, the levels of the input signals to the amplifier circuits 14a to 14c can be correspondingly changed to control the volume of the sounds emitted from the loudspeakers 15a to 15c.

FIG. 3 illustrates a modification of the FIG. 2 embodiment, in which the wiring for connection of the light-sensitive variable resistors 13a to 130 between the respective tone coloring circuits 12a to and amplifier circuits 14a to 140 is caused to be of minimum length in order to reduce noise production due to the mutual induction of the adjacent wires. To this end other optical fibers 17e to 17g extend out of the expression control apparatus 18 to the light-sensitive variable resistors 13a to 130, respectively, as in the FIG. 1 embodiment, in addition to the optical fibers 17a to 17c. The open ends of the optical fibers 17e to 17g are in confronting relation to those of the optical fibers 17a to 170, respectively, with the shutter 22-interposed therebetween. The shutter 22 is operatively associated with theexpression pedal 23 and has the three apertures 22a to 220 as in the FIG. 2 embodiment. Other details of configuration and operation of this third preferred embodiment of the invention are substantially as set forth above in connection with the embodiments of FIGS. 1 and 2.

FIGS. 4 to 7 inclusive illustrate by way of example the more practical construction of the expression control apparatus composed of the light-sensitive variable resistors 13, the optical fibers 17, the light source 19, the apertured shutter 22, and so forth. With particular reference to FIGS. 4 and 5, the reference numeral 30 in these drawings designates a mount positioned as at the front, lower portion of the console, now shown, of the electronic musical instrument.

A casing 31 fixedly installed on the mount 30 has a light source chamber 32 formed within its front portion, seen to the right in FIG. 5, for housing the light source 19 such as the lamp which is conveniently held by a socket 33 supported by the mount 30 for electrical connection to the aforesaid DC power supply 20 not shown in FIGS. 4 and 5. It is assumed that the light source chamber 32 is completely sealed against the leakage of the light emitted by the light source 19.

A row of appropriately spaced light-sensitive variable resistors 13a to are embedded in the rear surface of the casing 31, as will be understood by referring back to FIG. 2. It will further be seen that the light-receptive surfaces'of these"light-sensitive variable resistors are held opposite to light paths 34a to 34, respectively, formed through the casing 31, although only the light path 34b corresponding to the light-sensitive variable resistor 13b is seen in FIG. 5.

A vertical slot is formed at 35 through the casing 31 to receive the shutter 22 with appropriate clearance. As best shown in FIG. 6, this shutter 22 is formed with the apertures 22a to 220 corresponding to the respec- 7 tive light-sensitive variable resistors 13a to 130. The apertured shutter 22 has a tongue 36 projecting downwardly through the top 30a of the mount 30 to be somewhat loosely received in a slot 37a of a stationary guide 37. A helical compression spring is provided at 38 to resiliently urge the shutter 22 upwardly.

As will be apparent from FIG. 5, the aforesaid light paths 34a to 34c are aligned with light paths 39a to 390, respectively, through the vertical slot 35, although only the light path 39b aligned with the first mentioned light path 34b is seen in the drawing. Open to each of these light paths 39a to 39c is one end of each of the optical fibers 17a to 17c, and as best shown in FIG. 7 the other ends of the optical fibers project into the light source chamber 32 to be placed opposite to the light source 19. It may be noted from FIG. 7 that the said other ends of the optical fibers are located the same distance from the center of the light source 19 so that the light of equal intensity will be distributed into the respective optical fibers.

Referring back to FIGS. 4 and 5, a pair of arms 40a and 40b extend downwardly from the front portion of the expression pedal 23, that is, the portion on which the heel of the performers foot is to be placed. These arms are swingably coupled by a pin or rod 41 to brackets 42a and 42b, respectively, which are formed on both sides of the mount 30. The expression pedal 23 is thus made swingable on the pin 41. An abutment 43 is formed on the underside of the expression pedal 23 for contact with the top of the shutter 22, and a cushion 44 is provided on the top of the casing 31 to serve as a stop I limiting the downward swing of the expression pedal.

In the expression control apparatus'of the invention constructed as hereinbefore described with reference to FIGS. 4 to 7, the apertures 22a to 220 of the shutter 22 are all held out of alignment with the light paths 34a to 340 and 39a to 39c when the shutter is held in its elevated position by the helical compression spring 38, that is, when the expression pedal 23 is not depressed,

as will be seen from FIG. 4 in particular. As a consequence, the light distributed from the light source 19 is prevented from impinging onthe light-sensitive variable resistors 13a to 130, so that these variable resistors now offer substantially infinitely high electrical resistance.

If now it is assumed that the light-sensitive variable resistors 13a to 136 are electrically connected between the tone coloring circuits 12a to 12c and the amplifier circuits 14a to 140 respectively, as best represented by the embodiment of FIG. 2, then the output signals from the tone coloring circuits are attenuated by the high resistance of the respective light-sensitive variable resistors to such an extent that substantially no audible sound will be emitted from the loudspeakers 15a to 150.

As the expression pedal 23 is depressed to cause the shutter 22 to descend as indicated by the arrow in FIG. 5, the light from the light source 19 is permitted to irradiate the light-sensitive variable resistors 13a to 130 through the optical fibers 17a to 17c, the light paths 39a to 39c, the apertures 22a to 220 of the shutter 22, and the light paths 39a to 390, the apertures 22a to 22c of the shutter 22, and the light paths 34a to 340. It will be needless to say that the intensity of the light incident on the light-sensitive variable resistors increases as the expression pedal 23 is depressed deeper, and that the resistance of these light-sensitive variable resistors drops in inverse proportion to the intensity of the light 8 incident thereupon. Accordingly, the signals passing as aforesaid through the variable resistors are attenuated to a lesser degree, and the sounds are produced from the loudspeakers 15a to 15c with increasingly higher volume.

Hence, by the depression of the expression pedal 23 alone, the intensity of the light incident on the several light-sensitive variable resistors is regulatable simultaneously, with the resultant simultaneous control of the levels of the signals passing therethrough.

While in the exemplified expression control means of FIGS. 4 to 7 the shutter 22 is positioned between the optical fibers 17 and the light-sensitive variable resistors 13, it is of course possible to position the shutter between the light source 19 and the optical fibers as in the FIG. 1 embodiment or intermediate between the opposite ends of the optical fibers as in the FIG. 3 embodiment.

Moreover, while the optical fibers 17 are disposed separately in the casing 31, as best shown in FIG. 7, it will be selfevident that their ends opposite to the light source 19 can be combined as in the FIGS. 1 to 3 embodiments. It is also understood that not only one but a plurality of lamps or other light sources 19 can be provided within the light source chamber 32 of the casing.

Furthermore, although the intensity of the light incident upon the light-sensitive variable resistors in the above described example of the expression control apparatus is regulated by the apertured shutter associated with the expression pedal of the electronic musical instrument, this shutter can of course be actuated by a lever, a knob or any other suitable means which may be determined in relation with the particular application for which the invention is intended.

It must also be well noted that the utility of this invention is not limited to the expression control of the electronic musical instrument. Instead, the invention is adaptable for amplitude modulation to provide a tremolo effect or for frequency or phase modulation to provide a vibrato effect in the electronic musical instrument, for volume control of four-channel stereophonic sound reproduction systems or for other purposes where simultaneous control of signals being fed along separate paths in substantially parallel relationship is required.

All these and other modifications, substitutions and changes are intended in the foregoing disclosure, and in some instances some features of the invention may be employed without corresponding use of other features, while still remaining within the scope of this invention. It is therefore appropriate that the invention be construed broadly and in a manner consistent with the proper scope or fair meaning of the following claims.

We claim:

1. In an electronic musical instrument of the type comprising a plurality of keyboards, a plurality of tone coloring circuits provided correspondingly to said respective keyboards, each of said tone coloring circuits being adapted to impart a desired tone color to an electrical signal delivered thereto from a tone generator circuit each time any of the keys of the corresponding one of said keyboards is depressed, and a plurality of amplifier circuits the inputs of which are connected to the outputs of said respective tone coloring circuits and the outputs of which are connected to a plurality of loudspeakers respectively, an expression control sys-' tem comprising:

light source means;

a plurality of light-sensitive variable resistors connected between said tone coloring circuits and said amplifier circuits respectively, said light-sensitive variable resistors being caused to offer variable electrical resistance to the passage therethrough of the output signals from said respective tone coloring circuits by being supplied with the light of variable intensity from said light source means;

a plurality of optical fibers adapted to direct the light from said light source means to said respective light-sensitive variable resistors by internal reflection; and

means for regulating the intensity of the light directed from said light source means to said lightsensitive variable resistors by said respective optical fibers to cause the corresponding change in the volume of the sounds emitted from said loudspeakers.

2. An expression control system as recited in claim 1, wherein those ends of said optical fibers which lie opposite to said light source means are combined together to ensure that the light of equal intensity is delivered into said optical fibers.

3. An expression control system as recited in claim 1, wherein said means for regulating the intensity of the light include an apertured shutter positioned between said light source means and those ends of said optical fibers lying opposite to said light source means, the position of said apertured shutter relative to said light source means and said ends of said optical fibers being adjustable by means of an expression pedal of said electronic musical instrument.

4. An expression control system as recited in claim 1, whereinsaid means for regulating the intensity of the light include an apertured shutter positioned between said light-sensitive variable resistors and those ends of said optical fibers lying opposite to said light-sensitive variable resistors, the position of said apertured shutter relative to said light-sensitive variable resistors and said ends of said optical fibers being adjustable by means of an expression pedal of said electronic musical instrument.

5. An expression control system as recited in claim 1, wherein said means forregulating the intensity of the light include an apertured shutter positioned between the opposite ends of said optical fibers, the position of said apertured shutter relative to said optical fibers being adjustable by means of an expression pedal of said electronic musical instrument.

6. In an electronic musical instrument of the type comprising a plurality of keyboards, a plurality of tone coloring circuits provided correspondingly to said respective keyboards, each of said tone coloring circuits being adapted to impart a desired tone color to an electrical signal delivered thereto from a tone generator circuit each time any of the keys of the corresponding one of said keyboards is depressed, and a plurality of amplifier circuits the inputs of which are connected to the outputs of said respective tone coloring circuits and the outputs of which are connected to a plurality of loudspeakers respectively, an expression control apparatus comprising a casing defining a lightproof chamber therein;

light source means housed within said lightproof chamber;

a plurality of light-sensitive variable resistors embedded in said casing and electrically connected between said tone coloring circuits and said amplifier circuits respectively, said light-sensitive variable resisitors being caused to offer variable electrical resistance to the passage therethrough of the output signals from said respective tone coloring circuits by being supplied with the light of variable intensity from said light source means;

a plurality of optical fibers within said casing adapted to direct the light from said light source means to said respective light-sensitive variable resistors by internal reflection, each of said optical fibers having one end open to said lightproof chamber and the other end placed opposite to each of said lightsensitive variable resistors;

an apertured shutter slidably received in a slot formed in said casing between said light-sensitive variable resistors and said other ends of said optical fibers; and

an expression pedal swingably mounted above said casing for causing said apertured shutter to move relative to said light-sensitive variable resistors and said other ends of said optical fibers and hence for regulating the intensity of the light directed from said light source means to said light-sensitive variable resistors by said respective optical fibers to cause the corresponding change in the volume of the sounds emitted from said loudspeakers.

7. An expression control apparatus as recited in claim 6, wherein those ends of said optical fibers which are open to said lightproof chamber are so arranged relative to said light source means that said optical fibers are supplied with the light of equal intensity there- 

1. In an electronic musical instrument of the type comprising a plurality of keyboards, a plurality of tone coloring circuits provided correspondingly to said respective keyboards, each of said tone coloring circuits being adapted to impart a desired tone color to an electrical signal delivered thereto from a tone generator circuit each time any of the keys of the corresponding one of said keyboards is depressed, and a plurality of amplifier circuits the inputs of which are connected to the outputs of said respective tone coloring circuits and the outputs of which are connected to a plurality of loudspeakers respectively, an expression control system comprising: light source means; a plurality of light-sensitive variable resistors connected between said tone coloring circuits and said amplifier circuits respectively, said light-sensitive variable resistors being caused to offer variable electrical resistance to the passage therethrough of the output signals from said respective tone coloring circuits by being supplied with the light of variable intensity from said light source means; a plurality of optical fibers adapted to direct the light from said light source means to said respective light-sensitive variable resistors by internal reflection; and means for regulating the intensity of the light directed from said light source means to said light-sensitive variable resistors by said respective optical fibers to cause the corresponding change in the volume of the sounds emitted from said loudspeakers.
 2. An expression control system as recited in claim 1, wherein those ends of said optical fibers which lie opposite to said light source means are combined together to ensure that the light of equal intensity is delivered into said optical fibers.
 3. An expression control system as recited in claim 1, wherein said means for regulating the intensity of the light include an apertured shutter positioned between said light source means and those ends of said optical fibers lying opposite to said light source means, the position of said apertured shutter relative to said light source means and said ends of said optical fibers being adjustable by means of an expression pedal of said electronic musical instrument.
 4. An expression control system as recited in claim 1, wherein said means for regulating the intensity of the light include an apertured shutter positioned between said light-sensitive variable resistors and those ends of said optical fibers lying opposite to said light-sensitive variable resistors, the position of said apertured shutter relative to said light-sensitive variable resistors and said ends of said optical fibers being adjustable by means of an expression pedal of said electronic musical instrument.
 5. An expression control system as recited in claim 1, wherein said means for regulating the intensity of the light include an apertured shutter positioned between the opposite ends of said optical fibers, the position of said apertured shutter relative to said optical fibers being adjustable by means of an expression pedal of said electronic musical instrument.
 6. In an electronic musical instrument of the type comprising a plurality of keyboards, a plurality of tone coloring circuits provided correspondingly to said respective keyboards, each of said tone coloring circuits being adapted to impart a desired tone color to an electrical signal delivered thereto from a tone generator circuit each time any of the keys of the corresponding one of said keyboards is depressed, and a plurality of amplifier circuits the inputs of which are connected to the outputs of said respective tone coloring circuits and the outputs of which are connected to a plurality of loudspeakers respectively, an expression control apparatus comprising: a casing defining a lightproof chamber therein; light source means housed within said lightproof chamber; a plurality of light-sensitive variable resistors embedded in said casing and electrically connected between said tone coloring circuits and said amplifier circuits respectively, said light-sensitive variable resisitors being caused to offer variable electrical resistance to the passage therethrough of the output signals from said respective tone coloring circuits by being supplied with the light of variable intensity from said light source means; a plurality of optical fibers within said casing adapted to direct the light from said light source means to said respective light-sensitive variable resistors by internal reflection, each of said optical fibers having one end open to said lightproof chamber and the other end placed opposite to each of said light-sensitive variable resistors; an apertured shutter slidably received in a slot formed in said casing between said light-sensitive variable resistors and said other ends of said optical fibers; and an expression pedal swingably mounted above said casing for causing said apertured shutter to move relative to said light-sensitive variable resistors and said other ends of said optical fibers and hence for regulating the intensity of the light directed from said light source means to said light-sensitive variable resistors by said respective optical fibers to cause the corresponding change in the volume of the sounds emitted from said loudspeakers.
 7. An expression control apparatus as recited in claim 6, wherein those ends of said optical fibers which are open to said lightproof chamber are so arranged relative to said light source means that said optical fibers are supplied with the light of equal intensity therefrom. 